In one ingenious study of perception, researchers first gave 1-month-old infants a bumpy pacifier to feel in their mouth and only later allowed them to look at that pacifier – after they had placed it next to a smooth pacifier that they had not explored orally.

Cross-modal perception

Explanation:

Cross model perception is the perception in which people perceive the object in one sensory modality that they do have perceived previously in another modality. For example, when a child perceived a pacifier without seeing it and after some time start to see it as well as the example of cross-modal perception.  

Thus in the above context, the researcher first gave a one-month-old infant a bumpy pacifier that infant feel it through the mouth. After that infant has to allowed to look at pacifiers not explored orally is the example of cross-modal perception.

can you explain a little further maybe give multiple choice? ?

the answer is d.index